Regulatory standards and specifications for products and processes are often written based upon an existing technology. This is understandable as it is a practical and pragmatic approach. Often, the ideal technology may not be economical or achievable. The primary problem over time with these technology based standards is that they are infrequently revised as the technology changes unless there is some compelling reason to do so.
The forward driver vision requirements for vehicles are dictated by government regulations. The intent is to ensure that the driver has an adequate field of view to safely operate the vehicle. The ideal, based just upon human factors, would be to have an unobstructed view in all directions. This is not possible or practical. Given the structural requirements of the vehicle, the ideal must be compromised.
The process for determining the top of the driver vision zone is illustrated in FIG. 1. Regulations determine the minimum dimensions of the transparent portion 12 of the windshield. In the United States, the forward driver vision zones for passenger vehicles windshields are defined by SAE J903, Passenger Car Windshield Wiper Systems, which was first issued in 1964. J903 is reviewed every five years but has remained largely unchanged since its inception. The last update was in 1999.
At the time that J903 was first issued, windshields were typically mounted at a nearly vertical installation angle 108 and had little or no vertical curvature. Interestingly, several models produced from the mid-50s through the mid-60s were outfitted with what was then called a panoramic windshield. This windshield extended the vertical edges to wrap around and into the area typically occupied by the A-pillar. Due to the small radius of the wrap, the optical quality in this area was poor.
The windshield driver vision zones, per J903, are calculated with the windshield at installation angle 108. The installation angle 108 is the angle relative to horizontal 102 of a cord 104 connecting the top and bottom points of the vertical centerline 106. SAEJ903 calls for the use of a statistical analysis of a driver population to generate an elliptical shape that is defined by J903 as an “eyelipsise”. The eyelipsise bounds an area which will include the eye points for most of the population. The eye point 100 of a statistically tall driver is used to determine the top edge of the vision zone. Having defined the “eyelipsise” for the vehicle, J903 calls for creating a horizontal plane, passing through the tall driver eye point, and then rotating the plane about the driver eye point upward by 10 degrees. The intersection of the rotated plane and the windshield form the top of the driver vision zone. This imaginary line 14 is known as the AS1 line and is the boundary of the AS1 area. Government safety regulations require that this location be permanently marked on every windshield. The area below the AS1 mark must have light transmittance of at least 70% for vehicles sold in the United Sates, and must not be otherwise obscured by the rear view mirror, the black band 10, or any other objects. Vision zones for vehicles manufactured in the European Union must comply with ECE R43 which is similar to the US regulation with the one major exception which requires that the light transmittance below the AS1 line must be at least 75%.
The method for calculating the bottom of the zone is similar to the method used for the top but based upon the eye point of a statistically short driver and at a smaller rotation angle. The end result is a total minimum driver vision zone angle of less than 20 degrees. While this is adequate for most driving conditions, it can make it difficult to see high mounted signals and signs under some circumstances, such as when facing downhill, with such a limited view. In some parts of the world driver eye-level traffic signals are used in addition to the overhead signals for this reason.
The windshield shown in FIG. 1 has an installation angle of 23 degrees. It can be seen that the sheet metal 16 of the roof is tangent to the windshield at the top edge 15 of the windshield where the two meet. If the top edge 15 of the windshield were to be extended by just 10-15 cm, the driver vision could be increased from 10 degrees relative to horizontal to 45 degrees or better. A perspective view of the windshield and sheet metal 16 is shown in FIG. 2.
At one time, low windshield installation angles, as shown in FIG. 1, were reserved for bullet trains, high speed exotics and race cars. In response to rising fuel prices and government efficiency requirements, automotive manufacturers have been paying close attention to the aerodynamic drag factors of their vehicles. Vehicle losses due to aerodynamic drag increase exponentially relative to the speed of the vehicle and become substantial at higher speeds. The formula for calculating aerodynamic drag is:FD=½ρv2CDA WhereFD is the force of drag;ρ is the mass density of air;v is the velocity of the vehicle;A is the frontal area of the vehicle; andCD Is the drag coefficient, a number between 0 and 1 which takes into account the shape of the vehicles.
As the speed doubles, the losses quadruple. At 40 kph, the losses are 4 times what they are at 20 kph and at 80 kph they become 16 times what they are at 20 kph.
A vehicle with a flat box like front end might have a drag coefficient close to 1 while a high speed train or exotic car might be less than 0.3. The lower the value the more fuel efficient that vehicle will be. Thus, a windshield with a lower installation angle will be more efficient that one with a higher angle, with all else the same.
Designers have also been working to reduce or eliminate other sources of drag. The discontinuity between the edge of the glass and the sheet metal is a source of drag and also a source of cabin noise. This has driven the trend towards flush glazing and the elimination of decorative trim and moldings. The top edge 15 of a conventional windshield is one of the areas where turbulent energy consuming air flow occurs at the interface between the windshield and the sheet metal. Moving the interface into the roof-line would improve drag but this has not been commonly done.
The lower installation angle also allows for the sheet metal of the roof to be designed such that it is tangent to the windshield and has a large radii transition from the windshield to the largely horizontal roof. This makes it possible to produce a windshield that has a panoramic viewing angle in the vertical direction which will have good optics in the transition area and be possible and economical to fabricate using conventional glass bending processes.
In U.S. Pat. No. 6,118,410A, Nagy et. al extends the top edge 15 of the windshield in order to provide an electromagnetically transparent cover for an “antenna shelf” to be used for GPS, GSM, satellite and other signals rather than for vision or aerodynamics. The extended portion is not for driver vision and is covered by the black band 10.
The Opel Astra GTC vehicle, first produced in the 2005 model year, is one of the few examples of a series production vehicle with a panoramic windshield. The top edge 15 extended well into the roof line and above the drive and front seat passenger. As the windshield and the roof needed to be fabricated from the same glass that would meet the EU 75% light transmittance specification, a dark tint could not be used. The roof portion was equipped with a mechanic blind that added cost and weight and limited the glass functionality as it either blocked 100% the visible light or none, all shortcomings of